An ordinary sound-isolating earphone is configured as illustrated in FIG. 1, including an electroacoustic transducer 12 disposed inside a housing 11, a lead wire 125 which connects the electroacoustic transducer 12 to an external amplifier, for instance, a sound leading pipe 14 which transmits a sound wave generated by the electroacoustic transducer 12 to the external auditory canal, and an ear pad 15 which serves as a cushion when the earphone is inserted into the external auditory canal and also shuts off external noise.
The ear pad 15 having a sound outlet 16 at an extreme end of a portion inserted in the external auditory canal is made of soft plastic, rubber, or the like, having elasticity and fits in close contact with an inside wall of the external auditory canal without creating any gap. Consequently, the sound-isolating earphone constitutes an earplug structure as a whole. A sound-emitting portion of the electroacoustic transducer 12 is located in a closed space on a right side of a partitioning wall 13 as illustrated.
A sound-isolating earphone 2 can be securely fitted in the entrance of the external ear because the sound-isolating earphone 2 can be worn with the ear pad 15 inserted in the external auditory canal as illustrated in FIG. 2. Also, the ear pad 15 made of a material having flexibility can elastically deform with ease in accordance with the shape of the external auditory canal, making it possible to achieve a comfortable fit.
As a result, the sound-isolating earphone which is used by inserting the same in the entrance of the external auditory canal provides good acoustic isolation and high sound-sealing performance, so that external noise is less likely to be heard. This makes it possible to obtain high sound pressure sensitivity and hear a feeble sound even in a very noisy place. Also, this sound-isolating earphone provides an advantage that the same can easily be reduced in size and weight because the earphone is used by inserting the same in the entrance of the external auditory canal.
With the widespread use of portable music players in recent years, there is a growing demand for developing a sound-isolating earphone capable of outputting a high-quality sound.
Since the conventional sound-isolating earphone is structured to close off the external auditory canal, however, the state of resonance within the external auditory canal varies, causing a deviation of resonant frequency, between points in time before and after the earphone is fitted, and producing a serious defect with respect to frequency characteristics of the earphone.
Specifically, when the sound-isolating earphone is fitted as depicted in FIG. 2, there occurs a change in resonance mode because the earphone including the ear pad has the earplug structure which blocks the entrance of the external auditory canal as described in Patent Document 1. To be more specific, the resonance mode changes from one-end closed pipe resonance to both-end closed pipe resonance in which both ends are closed, the external auditory canal constituting a resonance box.
Consequently, as depicted in a graph of FIG. 3 representing sound pressure-frequency characteristics, the sound pressure at an eardrum position indicated by a broken line has peaks in ranges of 2.8 to 3.4 kHz and 8.5 to 10.2 kHz when the sound-isolating earphone is not fitted, whereas the peaks of the sound pressure at the eardrum position shift to positions in ranges of 5.7 to 6.8 kHz and 11.3 to 13.6 kHz as indicated by a solid line under the influence of closed-pipe resonance when the sound-isolating earphone is fitted.
For this reason, sound components at around 6 kHz are emphasized in the both-end closed pipe resonance mode when the sound-isolating earphone is fitted and, therefore, there has been a problem that a quasi-resonant state would be created, producing a buzzing echo sound.
To solve this problem, Patent Document 1 discloses a technique employing two isolated sound leading pipes having different path lengths as a sound leading portion which transfers a sound wave generated by an electroacoustic transducer of a sound-isolating earphone to the entrance of the external auditory canal. In this technique, two sound waves generated by the electroacoustic transducer and separately passed through the two sound leading pipes are recombined at an entrance of an external auditory canal to suppress the sound pressure of a frequency component of which half the wavelength equals a difference between the path lengths of the two sound leading pipes.
Also, Patent Document 2 discloses a technique employing an acoustic resistor (damper) mounted in a sound leading pipe so as to suppress high-frequency sound components with a capability to freely replace the acoustic resistor (damper) with a different one.